Typical master cylinders in use in automotive vehicles for many years are of the dual or tandem type; that is, they have two pressurizing chambers in a common bore having a closed end and an open end, with brake fluid being pressurized by a primary piston and a secondary piston arranged in tandem in the bore. One of the chambers is typically connected to one brake circuit, such as one containing the vehicle front brakes, and the other chamber is typically connected to another brake circuit containing the vehicle rear brakes. At times other circuit arrangements, and even additional pressurizing chambers, are used.
Such a master cylinder usually has a pair of holes in the main body adjacent each of the primary and secondary piston seals. The holes are connected with the master cylinder reservoir and extend to the bore. One of the holes is a compensating port located behind the seal of the associated piston. The other hole is a bypass hole positioned in front of the seal of the associated piston. When the master cylinder is actuated, the seals move across their respective bypass holes so that these holes are no longer fluidly connected with the pressurizing chambers, and the brake fluid in those chambers and the brake circuits connected with them can be pressurized to actuate the brakes. These seals are usually cup-like or V-block seals with lips which are subject to seal extrusion into the bypass holes under some conditions, resulting in damage to the seal and loss of full functioning pressure generation for the brake circuit associated with the damaged seal. In normal service braking operation, there is little pressure in the pressurizing chambers until the bypass holes are closed, so extrusion into those holes is minimized. However, with the advent of systems variously referred to as anti-lock, anti-skid, wheel lock control, or anti-block systems the modulation of brake pressures in the brake circuits when that type system is operating can cause rapid travel of the secondary piston seal back and forth across the front bypass hole. That is the bypass hole associated with the forward pressurizing chamber in the master cylinder bore.